In general, the durability of a rolling bearing is reduced if water enters the lubricant. For example, incorporation of as little as 100 ppm of water into the lubricant causes a reduction in bearing life by 32 to 48% (see Schatzberg, P. and Felsen, I. M., Wear, Vol. 12, pp. 331-342, "Effects of water and oxygen during rolling contact lubrication" (1968) and Schatzberg, P. and Felsen, I. M., Journal of Lubrication Technology, ASME Trans. F, 91, 2, pp. 301-307, "Influence of water on fatigue failure location and surface alteration during rolling contact lubrication" (1969)).
Therefore, a bearing used in an environment involving contact with water, such as a roll neck bearing of a rolling mill or a water pump bearing, should be given a countermeasure for sealing out water so as to avoid reduction of life. For example, JP-B-55-22648 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a roll neck bearing for a rolling mill with its both sides sealed by a contact seal, such as an oil seal, and JP-A-59-223103 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a bearing with its both sides sealed by a noncontact seal having a narrow bent gap.
The sealing structures disclosed have achieved improved sealing properties over conventional countermeasures but are still insufficient in view of the duration of life of a bearing. That is, the bearing having a contact seal cannot completely prevent water from seeping in when the temperature of the bearing drops because air within the bearing contracts and sucks in outside water. The bearing with a noncontact seal still cannot get rid of the problem of water's seeping in through the gap of the seal.
Seeing that the presence of only a little water as 100 ppm in the lubricant greatly influences the life of a bearing as stated above, any seal would produce no effect unless incorporation of water is completely prevented.
Since it has been unknown why existence of water causes such a significant reduction in bearing life (see Ioanniedes, E. and Jacobson, B., Ball Bearing Journal, Special '89, pp. 22-27 "Dirty lubricants-reduced bearing life" (1989)), it has been difficult to take a radical measure to extend the bearing life from the standpoint of material. Attempts to prevent water's seeping in have therefore been confined to superficial measures for improvement on the performance of a seal.